Which feature must be activated on a Cisco Secure Email Gateway to combat backscatter?

To combat backscatter, which is a type of spam that consists of bounce messages sent to forged sender addresses, the administrator must enable the Bounce Verification feature under Security Settings. This feature allows the appliance to verify whether a bounce message is legitimate or not by checking if the original message was sent from the appliance. If not, the bounce message is considered as backscatter and can be dropped or quarantined. References: [Cisco Secure Email Gateway Administrator Guide - Configuring Bounce Verification]